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On a local scale, Mytilus mussels dominate the intertidal to subtidal regions of rocky shores. M. edulis has the widest distribution pattern in the genus, extending from high intertidal to shallow subtidal (1–5 m) and even deeper waters, from estuarine to fully marine conditions, and from sheltered to extremely wave‐exposed shores. On rocky shores of open coasts, the species attaches to rock surfaces and crevices, and in sheltered harbours and estuaries it occurs on rocks and piers, often as dense masses of one or more layers, with individuals bound together by byssus threads. Young mussels colonise spaces within the bed, increasing spatial complexity and providing numerous niches for other organisms (Seed 1996). At exposed sites, the mussel prefers gently sloping, slow‐draining platforms to steep rock faces (Seed 1976). On such shores, dense beds of small mussels, often several layers thick, blanket the rocks (Figure 3.1). Local distributions on the upper and lower shores can be very erratic on a seasonal and annual basis, due to irregular cycles of settlement, competition, predation, wave action, exposure to air and desiccation, and detachment during storms (see later). Death of underlying individuals may detach the mussel bed from the substratum, leaving the bed vulnerable to tidal scour and wave action (Seed & Suchanek 1992). On sheltered shores and in estuaries, fewer but larger mussels are found (Figure 3.2). In these habitats, infaunal beds may occur on gravel or even quite sandy areas, although it is likely that some harder substratum embedded within more sandy areas is required. Subtidal populations on pier pilings, oil platforms and culture ropes enjoy a virtually predator‐free environment, and this, combined with continuous submersion, allows M. edulis to reach a large size in a relatively short period of time (e.g. 50 mm shell length in six to eight months; Page & Hubbard 1987). In its native range, M. galloprovincialis is found from exposed rocky outer coasts to sandy bottoms (Ceccherelli & Rossi 1984). As an invader, it typically requires rocky coastlines with a high rate of water flow. The mussel is able to outcompete and displace native mussels and becomes the dominant mussel species in certain localities. For example, survivorship and tolerance to air exposure and desiccation in M. galloprovincialis are reported to be double those of any of the indigenous species in South Africa (Branch & Steffani 2004). Experiments in which mussels were held for 42 weeks at the high tide mark, where they experienced seven days of continuous exposure to air, resulted in 92% survivorship in M. galloprovincialis compared to 78% for Perna perna, 37–46% for Choromytilus meridionalis and only 0–10% for Aulacomya atra. Also, M. galloprovincialis grows faster than native mussels and has a reproductive output between 20 and 200% greater than that of indigenous species (Van Erkom Schurink & Griffiths 1993).

Figure 3.1 Layers of small mussels (Mytilus) on barnacle‐covered rocks on a semi‐exposed shore at Clogherhead, Louth, Ireland.

Figure 3.2 A cluster of mussels (Mytilus) on a sheltered shore at Lough Hyne, Cork, Ireland. Mussels in this habitat can reach shell lengths of 50–60 mm or greater.

In New Zealand, P. canaliculus is distributed widely throughout the two main islands, but it is more common in the warmer northern parts of the country. Dense beds of up to 100 individuals m⁻² can be found in northern coastal areas, which can include rocky reefs, wharf piles and soft bottom habitats. Intertidal (mid‐littoral¹) populations are limited by aerial exposure, while subtidal (down to 50 m) populations are limited by predation pressure. Environmental parameters, such as temperature and salinity, are strong determinants of the distribution of this species. While its temperature range is from 5.3 °C in the south to 27 °C in the north, this mussel appears to prefer the warmer waters of bays and estuaries in northern coastal areas. A wide range of salinities are tolerated by the species, although the optimum range is 30–35 psu (Alfaro et al. 2011 and references therein). P. viridis mussels generally inhabit intertidal, subtidal and estuarine environments with high salinity. They form a thick carpetlike growth (up to 35 000 individuals m⁻²) on rocky surfaces and submerged structures like wharves, pilings, breakwaters and buoys, and they play a very important role in rocky shore ecosystems. The species colonises submerged rocks, wood, concrete, metal, old submerged logs, boats, PVC pipes, ropes, muddy sea bottoms and even seagrass beds and aerial roots of mangroves. P. viridis tolerates a temperature range of 15–32.5 °C without much problem. For example, the species can survive a temperature of 39 °C for about 200 min (Rajagopal et al. 1995), which is considerably greater than the thermal tolerance of P. perna (see later). Also, P. viridis thrives well at winter water temperatures as low as 12 °C and has a higher degree of adaptability to salinity changes than P. perna (Rajagopal et al. 2006).

P. perna, a native of tropical and subtropical rocky shores, colonises jetties, navigation buoys, petroleum platforms, wrecks and other artificial hard substrata. Hicks & McMahon (2002) found that the mussel’s long‐term, incipient lower and upper thermal limits were 7.5–30 °C, similar to the seasonal ambient water temperature range of 10–30 °C reported for other populations worldwide. These narrow incipient thermal limits, a low capacity for temperature acclimation and a poor freeze resistance may account for the mussel’s restriction to subtidal and lower eulittoral zones of cooler subtropical rocky shores. Adult salinity tolerance ranges from 19 to 44 psu, which is lower than that reported for a nonindigenous Texas population (15–55 psu) (GISD 2019).

The larvae of the ribbed mussel, Geukensia demissa, settle on subtidal oyster reefs, in intertidal salt marshes and on artificial structures in these habitats. Sometimes, they attach to one another in aggregations or to clumps of hollow grass stems (Spartina alterniflora) in low marshes. They are most abundant at the lowest shore levels within salt marshes, and occur in small numbers in the high marsh zone above the average high watermark (GISD 2015). The highest water temperature tolerated is 56 °C, and salinities are tolerated from nearly freshwater up to 70 psu, which is twice as salty as the open ocean (Cohen 2005).

Modiolus modiolus occurs, partly buried, in soft sediments or coarse grounds or attached to hard substrata, forming clumps or extensive beds or reefs. The mussel may be found on the lower shore in rock pools or in laminarian holdfasts but is more commonly found subtidally to a depth of ~280 m (Tyler‐Walters 2007). Dense populations of very young M. modiolus do occasionally occur subtidally in estuaries, although the species is more poorly adapted to fluctuating salinity than other mussel species (Bayne 1976). M. barbatus is found among rocks and stones on the lower shore, extending into the sublittoral to depths of around 100 m. Modiolus auriculatus is found attached to rocks and in rock crevices in the littoral and sublittoral zones to a depth of 25 m. It also inhabits sandy mudflats and sometimes intertidal reefs in rock pools and crevices. Abdel‐Razak et al. (2014) examined the distribution of the species over the intertidal zone, coastal fringing reefs, offshore reefs and lagoons along the Red Sea coast of Egypt. They identified several diverse habitats: flat and rocky with a few sandy depressions in both intertidal and subtidal zones, with moderate vegetation of macro algae and sea grass beds and dead and live small coral patches; mangrove trees and tidal zones that reach to about 100 m; and a substrate consisting of small gravel pieces mixed with sand, extending down to ~70 m, with a dense vegetation of macro algae and sea grasses.

In the Mediterranean, Brachidontes pharaonis is confined to high‐temperature and high‐salinity habitats, where it has established very high densities, completely covering rock surfaces at intertidal sites (Safriel et al. 1980; Sarà et al. 2008). Adult mussels show large temperature tolerances (9–31 °C) and occur at salinities from 35 to 53 psu. Lower winter temperatures limit their physiological activity (Galil 2006). B. exustus was believed to commonly inhabit rock pilings, seawalls and wharf pilings in the intertidal zone and was most abundant in the lower intertidal (Seed 1980). However, DNA barcoding has since revealed that B. exustus is actually a complex of five species (see earlier and Chapter 9). Perumytilus purpuratus dominates the mid and mid to high intertidal zones on rocky shores, forming dense three‐dimensional matrices that serve as a microhabitat for a wide variety of organisms. Consequently, these mussels have been defined as ecosystem bioengineers that play an important role in regional biodiversity (Pérez et al. 2008). Prado & Castilla (2006) tested the hypothesis that factors determining the habitat structural complexity and environmental heterogeneity of P. purpuratus matrices have significant effects on the associated macrofaunal community. They found 92 invertebrate taxa in P. purpuratus matrices. The number of layers (stratification) in the matrix had a significant effect on evenness (degree of similarity in abundance among the species): the greater the stratification, the lower the evenness index. Sediment retention by matrices in sheltered sectors had a significant effect on evenness: greater sediment retention resulted in lower evenness. Sediment retention also determined significant differences in macrofaunal assemblages. In matrices without sediment retention, mussel layering and the presence of algae on the shells of P. purpuratus determined significant macrofaunal differences. As a dominant competitor, P. purpuratus plays a major role in intertidal rocky shores where it is present, structuring communities and determining local biodiversity. Semimytilus algosus is another bioengineer mussel, cohabiting with P. purpuratus on most Chilean rocky shores. However, unlike P. purpuratus (see earlier), S. algosus occurs in the low intertidal zone and is a weak competitor of P. purpuratus, and S. algosus post‐settlers show high mobility to relocate in the intertidal, which may serve not only to allow this mussel to reach the optimal physiological position in the intertidal, but also to reduce competition interaction (Brante et al. 2019).

Arcuatula senhousia inhabits littoral and sublittoral rock and other hard substrata, and also hard and soft substrates in the intertidal and shallow subtidal zones to 20 m depth. The mussel may settle on hard surfaces, but prefers to settle gregariously on soft substrates, burrowing until only the hind part of the shell protrudes, after which the species secretes fibrous threads that attach to sediment particles to form a kind of nest or byssal cocoon around the shell. Gregarious settlement results in the formation of large colonies, raised several centimetres above the surface of the sand or mud (NIMPIS 2002). In some areas of California, the mussel has been reported to reach densities of 15 000 m⁻². The byssus mats formed by the mussel may smother native biota and can make the underlying seabed anoxic. A. senhousia is tolerant of low salinity and low oxygen levels. In San Francisco Bay, it has been collected at salinities of 17–33 psu and temperatures of 17–24 °C, and in southern California at 35–37 psu and 25–27 °C (Cohen 2005). Aulacomya atra is well known for its presence in deep water. For example, in Punta Arenas Cove, Chile, natural banks of the species are found at depths of 15 to >30 m, where temperatures vary from 12 to 16 °C. The mussel is also commonly found in shallow nearshore kelp‐bed communities and algae holdfasts in Central Chile, subantarctic and Antarctic waters, South Africa and cold and deep waters of the Kerguelen Islands (Caza et al. 2016). In South Africa, where A. atra is indigenous, its existence is compromised in many areas by the presence of alien species, such as M. galloprovincialis, which has a faster growth rate and a higher resistance to exposure to air on exposed and semi‐exposed shores compared to A. atra, which is predominantly a subtidal species (see later on interspecific competition). Choromytilus meridionalis is most abundant in the lower intertidal and subtidal zones and is geographically restricted to areas of cool upwelled water on the west and south coasts of South Africa. The mussel co‐occurs with P. perna on the southern coast of S. Africa. The two species exhibit a degree of habitat separation, with P. perna inhabiting sand‐free rocks and C. meridionalis inhabiting rocks subject to frequent sand cover (Marshall & McQuaid 1993).

Musculus discors is distributed from the shallow subtidal to depths up to 50 m in the British Isles. The mussel is found in scattered, gregarious clumps growing epiphytically on the holdfasts of seaweeds and among assemblages of attached animals growing on hard substrata. It occasionally forms extensive, dense aggregations covering upward‐facing rock surfaces (Tyler‐Walters 2001). Once attached, adults weave a ‘nest’ of several thousand fine byssus threads around their shells, so that they are suspended in a network of threads, like a ‘ball of twine’. The nest completely encloses the adult so that the mussel is only visible when its valves are open and it is feeding with siphons extended (Merrill & Turner 1963; see Chapter 5).

The charru mussel, Mytella charruana, is an epifaunal tropical (6–36 °C; Brodsky et al. 2009) and subtropical mussel. Its indigenous habitat is typically shallow lagoons and mudflats in bays, where it reaches high densities (de Oliveira et al. 2005 and references therein). Mussels are found tightly attached to a variety of naturally submerged and intertidal substrates, such as muddy sand, rocks, oyster shells, water intake pipes, wood pilings, driftwood floating docks and boat hulls. Juvenile and adult individuals are euryhaline, surviving at a wide range of salinities (2–40 psu; Yuan et al. 2010) at 20 °C, although the salinity tolerance range narrows as the temperature decreases or increases (Yuan et al. 2016b). Mytella guyanensis is a widespread euryhaline bivalve that occurs half‐buried in the intertidal zone of mangroves and estuaries from Baja California, Mexico to Santa Catarina, Brazil (de Oliveira et al. 2005).